RF coils play a crucial role in defining the spatial resolution of imaging systems, serving both to transmit excitatory RF signals to the tissues being interrogated and to collect the returning RF-signal information from the body. Magnetic resonance (MR) RF coils are essentially tuned inductors serving as antennae for the transmission or reception of RF signals.
RF coils are typically classified into two broad categories, volume coils and surface coils, differing in their reception characteristics and used in different imaging scenarios. Volume coils, which provide transmission and/or reception over a typically cylindrical or ellipsoidal region circumscribed by the coil, are designed to create RF fields that are, optimally, homogeneous throughout a specified region enclosed by the coil. A conventional birdcage coil is an example of a volume coil that may be used, for example, in neuroimaging applications. U.S. Pat. No. 4,692,705 to Hayes provides a description of such coils. Typically, a cylindrical version of a birdcage coil has a pair of circular end rings which are bridged by a plurality of equi-spaced straight segments or “legs,” each of which is interrupted by at least one reactive element. In a primary mode, currents in the legs should be sinusoidally distributed, which results in good uniformity of the distribution of the magnetic field produced by the coil (known as a B1-field) measured along the axis of the coil. The B1-field homogeneity may be improved by increasing the number of legs in the coil. In a typical variation of the birdcage design, additional capacitors may be distributed throughout the two end rings, thus interrupting the continuity of the end rings between adjacent legs, such as in a design described in U.S. Pat. No. 6,396,271 to Burl et al. In contrast to volume coils, surface coils are designed to operate efficiently over a limited spatial region of interest that is immediately adjacent to the coil.
Recent advances in RF coil design and a proliferation of specialized coils aim, in particular, at improving (i) the homogeneity of the magnetic field created by a coil, (ii) its quality factor (Q-factor), and (iii) tunability of its resonant frequency. In the art, the field homogeneity of the commercially available RF coils is generally considered to be satisfactory at fixed frequencies, lower level fields and with average constant loads.
A typical frequency-tuning range for commercially available coils is known to be restricted to 2-3 percent of the resonant frequency. This restriction imposes limitations on MR or NMR measurements. If a coil with extended range of tuning were available, it could allow for employing the same coil to obtain NMR signals from distinct NMR nuclei such as hydrogen isotopes and fluorine-19 labels, for example. A desirable tuning range for such operation is over 6% in order to encompass resonant frequencies of 600 MHz and 560 MHz in magnetic fields on the order of 14 Tesla. Wide tuning RF coil range (up to 30-40% of the resonance frequency) is of a particular interest for studying a variety of samples with different chemical compositions, dielectric (and other) looses, density, size etc.
One of the problems in achieving wide-range frequency tuning of an RF coil is the mutual dependence among the tuning range, the Q-factor, and the field homogeneity inside the coil. The resonant frequency ω of the RF-coil resonant circuit is determined by its inductance, L, and capacitance, C, as ω∝1/√{square root over (LC)}. Conventionally, birdcage resonators are tuned by making a small change to the value of resonating capacitors by, for example, connecting variable capacitors in parallel with fixed capacitors or by coupling a second, nearly resonant, circuit to the main coil and adjusting the reactance of the second circuit, typically by means of a variable capacitor. Lumped capacitances, however, make it difficult to achieve broad tuning range while maintaining electrical symmetry of the coil and, as a result, homogeneity of the generated magnetic field.
Burl et al, in U.S. Pat. No. 6,396,271, offered a design aiming to overcome the above-mentioned problems by providing a means for simultaneous tuning of multiple capacitors that interrupt the leg conductors of the coil. The proposed design, however, does not maintain the symmetry of the electrical configuration of the RF-coil circuit during the tuning procedure, which would be desirable to maintain the high value of the Q-factor and the magnetic field homogeneity inside the coil across the available tuning range. In addition, the proposed design faces a problem of local heating of the end ring capacitors. Added capacitors at either the end ring portions or at the leg portions of the RF coil are required. Clearly, then, the addition of multiple capacitors renders tuning the coil to the desired resonant frequency even less certain. The problem of tuning an RF coil across a wide range of frequencies while maintaining the field uniformity and the high Q-factor, therefore, still requires a solution.